Inorg. Chem. 1996, 35, 4529-4530
4529
Formation of Explosive Chlorine-Nitrogen Compounds during the Reaction of Ammonium Compounds with Chlorine M. Knothe* Freiberger NE-Metall GmbH, Lessingstrasse 41, 09599 Freiberg, Germany
W. Hasenpusch Degussa, AG, Abteilung Umweltschutz, 60287 Frankfurt (Main), Germany ReceiVed NoVember 22, 1995 Introduction It is known that during the reaction of chlorine or hypochloride with ammonium compounds chlorine amines of the composition NH3-nCln (n ) 0-3) may be formed as unstable intermediate compounds. Of them, NCl3 is especially explosive.1 The following systems are considered to be dangerous:2 (i) amounts of g0.5 mL of liquid NCl3; (ii) mixtures with g10 vol % of NCl3 in inert solvents, e.g., chlorinated hydrocarbons; (iii) mixtures with g0.5 vol % of NCl3 in the gas phases. The literature data concerning the formation of NCl3 during the reaction of NH4Cl with chlorine
NH4Cl + 3Cl2 ) NCl3 + 4HCl
(1)
are dispersed and, in part, incomplete.2 This especially applies to the dependence of NCl3 formation upon acidity. In the weakly acidic medium (pH 1-6) NCl3 is rapidly formed at reaction yields from 30 up to 70%. At pH g 10 and CHCl > 6 M, no NCl3 is formed. As to the acidity range from 0.1 to 6 M there are not sufficient data. For this range NCl3 formation was studied with the use of higher NH4Cl concentration and at different reaction temperatures. Experimental Section The experiments were performed with the use of 100-500 mL of solution. Because of the great influence of apparatus conditions on NCl3 formation the dependence upon relevant experimental parameters was always determined with the same apparatus conditions. In View of the tendency of NCl3 to explode, the following precautions were taken: assembly of the apparatus behind double glass shields; avoidance of critical phase composition by continuously agitating the reaction solution and passing an air flow through the free reaction space. For a given time chlorine was passed through the reaction solution under intensive agitation with, at the same time, a given air flow maintained in the gas space. The chlorine stream was determined, and from that the chlorine quantity introduced into the system was calculated. The reaction gases were passed through a system of washing bottles. The fractions were processed after the completion of chlorination and a downtime of 5 min. The quantities of NCl3 passed from the reaction solution into the gas phase as well as the NCl3 content in the reaction solution after the experiment had been finished were determined. The total NCl3 quantity was given in millimoles of NCl3 as well as in fractions of the added quantities of NH4Cl and chlorine according to eq 1. In addition, the remaining NH4Cl content in the reaction solution after the separation of NCl3 was determined. (1) Gmelin, Handbuch der anorganischen Chemie; Verlag Chemie: Weinheim, Germany: (a) System No. 6, Chlorine (Hauptband, 1927; Erga¨nzungsband, 1969); (b) System No. 23, Ammonium, 1936. (2) Dokter, T. J. Hazard. Mater. 1985, 12, 207.
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Figure 1. Dependence of NCl3 formation upon the acidity of reaction solution: (Curve 1) NCl3 quantity formed (mmol); (curve 2) NCl3 quantity formed relative to the NH4Cl quantity used (%); (curve 3) NCl3 concentration in the reaction solution (mmol/L). The determination of NCl3 was based on the selective separation of NCl3 (a) from the reaction solution by extraction with CCl4 followed by conversion with hydrochloric acid to NH4Cl and (b) from the gas phase by absorption in 12 M hydrochloric acid. The final determination of NH4+ was performed photometrically.3 The detection limits for NCl3 were in the gas phase 0.007 mmol and in the reaction solution 0.0035 mmol of NCl3. The correctness of the value obtained was verified by determining the blank value and by model tests using a 0.9 M NCl3 solution in CCl4.
Results Preliminary tests have also shown that in clearly acidic solutions relevant quantities of NCl3 have rapidly formed, being highly dependent on the apparatus conditions and, thereby, on the intensity of interaction between the chlorine and solution phases (during the reaction of 0.5 M NH4Cl/0.5 M HCl solution with chlorine the yield varied between 30 and 65%). Under equal conditions the formed NCl3 quantity increased nearly proportionally to the chlorine flow and, thereby, to the chlorine quantity introduced. Dependence of NCl3 Formation upon the Acidity of Solution. Under constant conditions (using each time 0.1 L of 0.5 M NH4Cl solution of different acidity, reaction temperature 20 °C, chlorination time 30 min, chlorine flow 8-9 L/h, and chlorine quantity added 180-206 mmol corresponding to 120137% according to eq 1) the dependence of NCl3 formation upon the acidity was determined within the range from pH ) 4.6 to 6.8 M. The results are shown in Figure 1. Influence of NH4+ Concentration. Under constant conditions (using 0.1 L of 0.5 M HCl solution, reaction temperature 20 °C, chlorination time 30 min, chlorine flow 8.5 L/h, and chlorine quantity added 185 mmol) the NH4Cl concentration varied from 0.01 to 2 M. The results are shown in Figure 2. Influence of Reaction Temperature. Under constant conditions (using 0.1 L of 0.5 M NH4Cl solution in 1.97 M HCl, chlorination time 30 min) at two different chlorination rates the reaction temperature was varied as follows. Series A: 5 L/h of Cl2; chlorine quantity 113 mmol corresponding to 73% according to eq 1. Series B: 9.0 L/h of Cl2; chlorine quantity 203 mmol corresponding to 132% of the theoretical value. The results are shown in Figure 3. (3) De Vries, T.; Savariar, C. P.; Chakrabartty, M. M. J. Am. Water Works Assoc. 1962, 54, 858.
© 1996 American Chemical Society
4530 Inorganic Chemistry, Vol. 35, No. 15, 1996
Notes Table 1. Dependence of Formation and Decomposition Reaction of NCl3 upon Timea reacn solution time, min
Figure 2. Dependence of NCl3 formation upon NH4Cl concentration in the reaction solution: (Curve 1) NCl3 quantity formed (mmol); (curve 2) NCl3 quantity formed relative to the NH4Cl quantity used (%); (curve 3) NCl3 concentration in the reaction solution (mmol/L).
Figure 3. Dependence of NCl3 formation upon reaction temperature: (Curve 1) chlorine quantity used, 73% of the theoretical value; (curve 2) chlorine quantity used, 132% of the theoretical value.
Time Dependence of the Formation and Decomposition Reactions of NCl3. With the use of 0.5 M NH4Cl solution in 0.5 M hydrochloric acid the formation rate of NCl3 was studied for a period of 45 min. For that, samples were taken and analyzed from both the reaction solution and the absorption solution arranged behind that. After the chlorine flow had been shut off, agitating the chlorine-containing reaction solution and passing the carrier gas flow through the free reaction space were continued. The results are given in Table 1. From that the following can be seen: (i) In the reaction solution a rapid formation of NCl3 took place with the greater portion of NCl3 (93-96%) initially remaining in the aqueous solution. (ii) After the chlorine flow had been shut off, the NCl3 quantity in the reaction solution decreased. Of that, 77% was recovered in the absorption solution. (iii) Of the NCl3 quantity removed subsequently from the reaction solution during 21 h, only 16% was recovered in the absorption solution.
CNCl3, mmol/L
nNCl3, mmol
abs solution nNCl3, mmol
10 20 30 45
Chlorination Reaction 4.4 2.21 12.9 6.33 17.1 8.29 26.4 12.50
0.09 0.31 0.67 2.1
15 30 45 60 180 24 h
Decomposition Reaction 22.9 10.50 12.9 5.71 11.4 4.93 8.1 3.41 1.57 0.64 0.019 0.007
n.b.c n.b. n.b. n.b. 10.4 0.1b
a Quantity used: 0.5 L of 0.5 M NH4Cl/0.5 M HCl. Specific chlorine flow: 5.4 L/(h‚L of solution). b The absorption solution was changed. c No values.
Summary By the reaction of chlorine with NH4Cl solutions (investigated concentration range 0.01-2 M NH4Cl), a rapid formation of NCl3 took place. The acidities at which the formation of relevant NCl3 quantities must be expected ranged from neutral values up to a hydrochloric acid concentration of about 5 M. Rising temperatures, indeed, diminished the NCl3 formation; however, even at about 90 °C noticeable NCl3 quantities were still formed. The behavior of NCl3-containing systems was governed by the noticeable, but limited, solubility of the NCl3 in aqueous solution and its relatively high vapor pressure and remarkable chemical stability against excessive NH4Cl. Therefore, during the formation stage the NCl3 concentration in the reaction solution initially increased which is accompanied with a relatively low transition of the NCl3 into the gas phase. Then, the NCl3 concentration in the gas phase distinctly increases when the solubility limit of NCl3 in the aqueous solution (about 17 mmol/L) is exceeded. When the flow of chlorine had been shut off, the NCl3 content in the reaction solution considerably decreases within some hours. The decrease is due, mainly, to the evaporation of NCl3 and removal by the gas phase, whereas the reaction with water or NH4Cl present in a large excess contributes only a little to this. The reaction resulting in stable compounds such as N2 and NO3- became decisive only at low NCl3 contents. Safety Precautions From the investigations it follows that during the reaction of NH4Cl solutions with chlorine the quantity of NCl3 formed as well as the NCl3 concentrations in many cases pass beyond the limit of dangerous conditions. In view of the explosion hazard caused already by small NCl3 quantities,2 all reactions in which chlorine and hypochloride, on the one hand, and amines, on the other, are taking part must be tested for the possibility of forming explosive chlorine amines. IC951503M
